This invention relates to improvements in an exhaust gas purifying system for purifying exhaust gas discharged from an internal combustion engine (gasoline-fueled or diesel) of an automotive vehicle or a boiler, regarding noxious components such as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx), and more particularly to the exhaust gas purifying system for effectively removing NOx, HC and CO in an oxygen-excessive region (lean region) or a low temperature region of exhaust gas.
In recent years, automotive vehicles of a low fuel consumption have been eagerly desired from the view points of exhaustion of petroleum resource and warming-up phenomena of the earth. Regarding automotive vehicles with a gasoline-fueled engine, attention has been paid on development of automotive vehicles provided with a so-called lean-burn engine. In the automotive vehicles provided with the lean-burn engine, exhaust gas is in an oxygen-excessive or lean region (atmosphere) in which an air-fuel (air/fuel) ratio is leaner or larger than a stoichiometric value. In case that a usual three-way catalyst is used in the lean region, removing or reducing NOx can be insufficient under the effect of the excessive oxygen. Accordingly, development of a catalyst for effectively reducing NOx even under an oxygen-excessive condition has been desired.
A catalyst for reducing NOx in such a lean region has been proposed as disclosed in Japanese Patent Provisional Publication No. 5-168860 in which Pt and lanthanum are carried on a porous carrier so that NOx is adsorbed in the lean region and released in a stochiometric region in which exhaust gas has a stoichiometric air-fuel (air/fuel) ratio. However, there is the problem that a sufficient NOx reducing effect cannot be obtained even if such a catalyst is used.
Otherwise, the three-way catalyst has hitherto been used to simultaneously carry out oxidation of CO and HC and reduction of NOx in exhaust gas from a gasoline-fueled engine. A representative example of such a three-way catalyst is produced by forming a carrier layer of y-alumina on a heat-resistant substrate of cordierite or the like, and by causing catalytic noble metals such as platinum (Pt), palladium (Pd) and rhodium (Rh) to be carried on the carrier layer.
Now, from the viewpoint of protection for global environment, carbon dioxide (CO2) in exhaust gas discharged from the internal combustion engine of an automotive vehicle or the like is problematic. In view of this, the lean-burn engine for accomplish lean-burn in the lean region and diesel engine seem to be full of promise. These engines can improve the fuel consumption or economy of the automotive vehicle under the effect of reduction of fuel to be consumed thereby suppressing generation of CO2 as a combustion or exhaust gas from the engine. Such a catalyst for removing HC, CO and NOx in the lean region is disclosed in Japanese Patent Provisional Publication No. 9-57098. In this catalyst, Pt and Rh are carried separate from Pd so that NOx is reduced under the action of Pt upon being adsorbed or trapped in a NOx adsorbing or trapping material while HC and CO in a stochiometric or rich region is oxidized under the action of Pd.
However, the above NOx adsorbing material is not advantageous for adsorption or trap of NOx in a considerably low temperature region (not higher than 140xc2x0 C.), and therefore there is the possibility of lowering the absorption amount of NOx. Additionally, it has been known that the catalytic activity of the catalyst varies according to the kinds of the catalytic noble metals to be used. Additionally, Pd tends to be easily poisoned with sulfur compounds existing in exhaust gas, as compared with Pt. Further, it has been known that NO, HC and the like coexisting in exhaust gas impede the catalytic activity for oxidation of CO in the considerably low temperature region of exhaust gas.
By the way, it is conventional that diesel engines or the like are equipped with a catalyst disposed in an exhaust gas passageway so as to remove or reduce NOx discharged from the engine. Known examples of such a catalyst are zeolite-based one, alumina-based one and the like; however, they can exhibit a NOx reducing effect only within a limited temperature range. In view of this, it has been proposed to combine a plurality of catalysts which have different temperature ranges in which the NOx reducing effect can be exhibited, for the purpose of extending the temperature range exhibiting the NOx reducing effect. For example, Japanese Patent Provisional Publication No. 6-134258 discloses an exhaust gas purifying system which includes a plurality of catalysts which have different temperatures at which the maximum catalytic activities are obtained, upon varying the amount of cobalt carried on mordenite of the catalysts. It is usual to dispose the catalyst having a relatively high temperature activity on the upstream-side of the catalyst having a relatively low temperature activity in the exhaust gas passageway. Additionally, Japanese Patent Provisional Publication No. 6-307231 discloses an exhaust gas purifying system including a plurality of NOx reducing catalysts which are arranged in series relative to flow of exhaust gas and such that their oxidation activity for hydrocarbons increases as their location shifts to the downstream-side.
Thus, although the temperature range exhibiting the NOx reducing effect of the exhaust gas purifying system can be extended by combining a plurality of NOx reducing catalysts, a NOx reducing efficiency is low at a part where curves of the NOx reducing effects of the plural NOx reducing catalysts cross, forming a trough in NOx reducing efficiency within a temperature range. Accordingly, even by combining the plural NOx reducing catalysts, a practically sufficient NOx reducing efficiency cannot be obtained for the reason of existence of the NOx reducing efficiency trough within the temperature range, which is largely problematic. Particularly within a low temperature range at which the catalytic activity of the catalysts is inherently low, the amounts of NOx to be reduced by the catalysts are small even upon using the plural catalysts, and therefore the NOx reducing efficiencies of the catalysts are sharply lowered.
In view of the above background, the present inventors have found such a knowledge that a multistage catalyst system cannot achieve a high improvement in NOx reducing efficiency, because reducing agents (such as CO and HC) highly useful in NOx reduction in a high temperature range impede NOx reduction in a low temperature range.
It is, therefore, an object of the present invention to provide an improved exhaust gas purifying system for an internal combustion engine, which can overcome drawbacks encountered in conventional exhaust gas purifying systems including catalysts for purifying exhaust gas.
Another object of the present invention is to provide an improved exhaust gas purifying system for an internal combustion engine, which can effectively remove or reduce NOx in exhaust gas in an oxygen-excessive or lean region, discharged from the engine by regulating the concentrations of the gas components of exhaust gas.
A further object of the present invention is to provide an improved exhaust gas purifying system for an internal combustion engine, which can effectively remove or oxidize CO and HC in exhaust gas even in a low temperature region by regulating adsorption and release of NOx in accordance with the temperature of exhaust gas.
A still further object of the present invention is to provide an improved exhaust gas purifying system for an internal combustion engine, which can effectively remove or reduce NOx in exhaust gas in an oxygen-excessive or lean region, discharged from the engine by regulating the concentrations of the gas components of exhaust gas, while effectively removing or oxidizing CO and HC in exhaust gas even in a low temperature region by regulating adsorption and release of NOx in accordance with the temperature of exhaust gas.
A still further object of the present invention is to provide an improved exhaust gas purifying system for an internal combustion engine, which can exhibit a high NOx removing efficiency throughout a wide range of exhaust gas temperature, particularly in a low exhaust gas temperature range.
An aspect of the present invention resides in an exhaust gas purifying system for an internal combustion engine, comprising an exhaust gas component concentration regulating device disposed in an exhaust gas passageway of the engine, for regulating concentrations of gas components in exhaust gas discharged from the engine such that the concentrations of carbon monoxide and hydrogen are respectively not more than 2.0% by volume and not less than 0.5 by volume and such that a volume concentration ratio of [hydrogen/carbon monoxide] is not smaller than 0.5, in a first exhaust gas condition in which air/fuel ratio of exhaust gas is within a range of from a rich value and a stoichiometric value. Additionally, a NOx adsorbing and reducing catalyst is disposed in the exhaust gas passageway downstream of the exhaust gas component concentration regulating device, for adsorbing nitrogen oxides in a second exhaust gas condition in which the air/fuel ratio of exhaust gas is at a lean value, and reducing the nitrogen oxides into nitrogen in the first exhaust gas condition.
Another aspect of the present invention resides in an exhaust gas purifying system for an internal combustion engine, comprising a NOx adsorbing catalyst disposed in an exhaust gas passageway of the engine, the NOx adsorbing catalyst adapted to adsorb and release nitrogen oxides in accordance with a temperature of exhaust gas, the NOx adsorbing catalyst adapted to adsorb nitrogen oxides in exhaust gas in a first exhaust gas temperature range in which exhaust gas has a temperature ranging from a level at engine starting to 140xc2x0 C. and release the adsorbed nitrogen oxides in a second exhaust gas temperature range in which exhaust gas has a temperature of not lower than 200xc2x0 C. Exhaust gas discharged from the NOx adsorbing catalyst has a volume concentration ratio of [nitrogen oxides/carbon monoxide] which is not larger than 0.3 in the first exhaust gas temperature range. Additionally, an oxidizing catalyst is disposed in the exhaust gas passageway downstream of the NOx adsorbing catalyst, for oxidizing oxidizable components in exhaust gas discharged from the NOx adsorbing catalyst.
A further aspect of the present invention resides in a NOx adsorbing catalyst used in an exhaust gas purifying system for an internal combustion engine. The NOx adsorbing catalyst disposed in an exhaust gas passageway of the engine and adapted to adsorb and release nitrogen oxides in exhaust gas in accordance with a temperature of exhaust gas. The exhaust gas purifying system includes an oxidizing catalyst in the exhaust gas passageway downstream of the NOx adsorbing catalyst, for oxidizing oxidizable components in exhaust gas discharged from the NOx adsorbing catalyst. In the above exhaust gas purifying system, the NOx adsorbing catalyst is adapted to adsorb nitrogen oxides in exhaust gas in a first exhaust gas temperature range in which exhaust gas has a temperature ranging from a level at engine starting to 140xc2x0 C. and release the adsorbed nitrogen oxides in a second exhaust gas temperature condition in which exhaust gas has a temperature of not lower than 200xc2x0 C. Exhaust gas discharged from the NOx adsorbing catalyst has a volume concentration ratio of [nitrogen oxides/carbon monoxide] which is not larger than 0.3 in the first exhaust gas temperature range.
A still further aspect of the present invention resides in an exhaust gas purifying system for an internal combustion engine, comprising an exhaust gas component regulating device disposed in an exhaust gas passageway of the engine, at least one of a NOx adsorbing and reducing catalyst and a NOx adsorbing catalyst disposed in the exhaust gas passageway downstream of the exhaust gas component regulating device, and an oxidizing catalyst in the exhaust gas passageway downstream of the at least one of the NOx adsorbing and reducing catalyst and the NOx adsorbing catalyst. In the above exhaust gas purifying system, the exhaust gas component concentration regulating device is adapted to regulate concentrations of gas components in exhaust gas discharged from the engine such that the concentrations of carbon monoxide and hydrogen are respectively not more than 2.0% by volume and not less than 0.5% by volume and such that a volume concentration ratio of [hydrogen/carbon monoxide] is not smaller than 0.5, in a first exhaust gas condition in which air/fuel ratio of exhaust gas is within a range of from a rich value and a stoichiometric value; the NOx adsorbing and reducing catalyst is adapted to adsorb nitrogen oxides in a second exhaust gas condition in which the air/fuel ratio of exhaust gas is at a lean value, and reducing the nitrogen oxides into nitrogen in the first exhaust gas condition; the NOx adsorbing catalyst is adapted to adsorb and release nitrogen oxides in accordance with a temperature of exhaust gas, the NOx adsorbing catalyst adapted to adsorb nitrogen oxides in exhaust gas in a first exhaust gas temperature range in which exhaust gas has a temperature ranging from a level at engine starting to 140xc2x0 C. and release the adsorbed nitrogen oxides in a second exhaust gas temperature range in which exhaust gas has a temperature of not lower than 200xc2x0 C., exhaust gas discharged from the NOx adsorbing catalyst having a volume concentration ratio of [nitrogen oxides/carbon monoxide] which is not larger than 0.3 in the first exhaust gas temperature range; and the oxidizing catalyst is adapted to oxidize oxidizable components in exhaust gas discharged from the at least one of the NOx adsorbing and reducing catalyst and the NOx adsorbing catalyst.